1. Field of the Invention
The present invention relates to a recording and reproduction apparatus. More particularly, the present invention relates to a recording and reproduction apparatus in which the average driving power of a stepping motor for moving an optical head in a recording and reproduction apparatus, such as an optical disk apparatus, is reduced and in which an occurrence of what is commonly called track jumping caused by vibrations when the stepping motor is rotationally driven is prevented.
2. Description of the Related Art
In a recording and reproduction apparatus, such as an optical disk apparatus, generally a stepping motor is used as a driving source for moving an optical head for recording or reproduction (hereinafter referred to simply as a head) in the radial direction of a disk and for stopping it at a position near a target track so as to perform what is commonly called microstep driving. Hereinafter, a conventional recording and reproduction apparatus will be described with reference to FIGS. 5 to 8 using an example of an optical disk apparatus.
FIG. 5 is a block diagram illustrating the essential portion of an optical disk apparatus. FIG. 6 shows a current control signal for controlling the driving current of a stepping motor. FIGS. 7 and 8 show the driving current of the stepping motor. As shown in FIG. 5, the optical disk apparatus is provided with an optical disk 2 which rotates by a spindle motor 1, a head 4 which causes an optical beam to track the tracks of the optical disk 2 by the movement of a lens 3 or the like, and a track servo section 5 which performs tracking servo of the head 4 in accordance with a track error signal TES indicating the track position deviation of the head 4. This track servo section 5 generates a tracking servo signal TSV in accordance with the track error signal TES and a control signal from a control circuit 6 in order to servo control the head 4.
On the optical disk 2, signals are recorded on tracks formed in a spiral shape from the inner region of the disk to the outer region, and an optical beam is radiated onto these tracks. The movable range of the optical beam radiated onto the optical disk 2 in the track direction is several tens of tracks by only the head 4 and tracking servo control, and it is impossible to access the entire surface of the optical disk 2. For this reason, driving means for moving and positioning the head 4 in place is provided to move and position (referred to as coarse movement) the head 4 near a target track. As this driving means, a stepping motor 7 which is efficient and can be controlled by an open loop is generally used. The head 4 moves across the tracks in the radial direction of the optical disk 2 which is rotated by the spindle motor 1 by this stepping motor 7. This movement is controlled in response to the rotational control of the stepping motor 7 as a result of the engagement of the needles (not shown) of the head 4 with the lead screw (not shown) formed on the rotation shaft of the stepping motor 7.
The stepping motor 7 is driven by driving currents Ca and Cb from a driving circuit 8 which is driving current supply means. This driving circuit 8 generates the driving currents Ca and Cb in the stepping motor 7 in accordance with current control signals Sa and Sb from the control circuit 6 and polarity switching signals Pa and Pb for controlling the switching of the polarity of the driving current. Here, the stepping motor 7 is what is commonly called microstep-driven. Since the rotation torque must always be constant, the driving currents Ca and Cb are driven by a current which varies in a Sin or Cos fashion: Hereinafter, a case in which a stepping motor of two-phase (A phase and B phase) driving is driven at 1 step angle in 4 micro steps will be described with reference to FIGS. 6 to 8.
The current control signals Sa and Sb, such as those shown in FIG. 6, whose levels increase or decrease in a stepwise manner for each micro step and which vary in a Sin or Cos fashion as a whole, are supplied from the control circuit 6 to the driving circuit 8 in order to microstep-drive the stepping motor 7. Here, Sa is the current control signal of A phase, and Sb is the current control signal of B phase. Further, the current control signals Sa and Sb are shown until the rotational angle of the stepping motor 7 is 180.degree., but over 180.degree. the signals repeat. Meanwhile, the control circuit 6 supplies the polarity switching signals Pa and Pb for switching the polarity of the driving current to the driving circuit 8 according to the rotational angle of the stepping motor 7. For example, the polarity switching signal Pa of A phase controls the driving circuit 8 so that a driving current in a positive (+) direction flows from 0.degree. to 180.degree., and a driving current in a negative (-) direction flows from 180.degree. to 360.degree.. The polarity switching signal Pb of B phase controls the driving circuit 8 so that a driving current in a positive (+) direction flows from 0.degree. to 90.degree., a driving current in a negative (-) direction flows from 90.degree. to 270.degree., and a driving current in a positive (+) direction flows again from 270.degree. to 360.degree.. These current control signals Sa and Sb and polarity switching signals Pa and Pb are generated by a digital circuit (not shown) within the control circuit 6, and the rise and fall of the signal waveform are sharp.
FIG. 7 shows the waveform of the driving current supplied from the driving circuit 8 to the stepping motor 7 up to one rotation (360.degree.). Ca is the driving current of A phase, and Cb is the driving current of B phase. As can be seen in FIG. 7, the driving currents Ca and Cb flow in a positive (+) direction or in a negative (-) direction. This is due to the polarity switching signals Pa and Pb from the control circuit 6. Except for this, the signals are analogous to the current control signals Sa and Sb of FIG. 6, and are in a Sin or Cos fashion as a whole. The magnitude C (see FIG. 7) of the driving current supplied in an interval T of each micro step corresponds to the level S of the current control signal of FIG. 6. The reason why the signals become analogous is that the inductance of the excitation coil of the stepping motor 7 is very small. Therefore, the sum (total current) of the driving current supplied from the driving circuit 8 to the stepping motor 7 is that shown in FIG. 8, and an electric current of more than a maximum value (electric-current reading 1.0 in FIG. 7) of the driving current which is made to flow to one phase always flows.
As described above, the stepping motor 7 is highly efficient and can be driven by open-loop control, which is advantageous in constructing the apparatus. However, during coarse movement, since the head 4 must be moved near to a target track in as short a time as possible, the stepping motor 7 is always microstep-driven with a large current, having the drawback of causing an increase in the consumption of power. Further, the driving circuit 8 requires a switching element and the like which withstands a large current, resulting in an increase in the cost of the apparatus. Furthermore, a heat sink is required due to the heat generation in the stepping motor 7 and the driving circuit 8, hindering a reduction in the size of the apparatus and causing an increase in the cost.
Because of coarse movement, when the driving current is shut off to stop the head 4 at the place where it reaches a predetermined position, the rotation shaft of the stepping motor 7 is pressingly returned to the detent neutral point by the detent torque which is characteristic of microstep driving. Vibrations which occur at this time are transmitted to the head 4, the lens 3 and the like within the head 4 fluctuates, causing what is commonly called track jumping and the light spot deviates from the track. If this deviation is large, the problem arises that the data error rate increases, and tracking servo is interrupted; making servo impossible.
Further, the waveforms of the driving currents Ca and Cb for rotating the stepping motor 7 are analogous to the waveforms of the current control signals Sa and Sb. These current control signals Sa and Sb are generated by a digital circuit (not shown) of the control circuit 6, and the rise and fall of the waveform are sharp. For this reason, the waveforms of the driving currents Ca and Cb have sharp rise and fall, and a large acceleration is applied to the head 4 during coarse movement, causing the lens 3 and the like within the head 4 to vibrate and similar track jumping occurs.